1. Field of the Invention
The present invention relates to boots for protecting constant velocity universal joints disposed, for example, on a driving shaft (power transmission shaft) or the like which transmits a power to an axle hub from a differential gear of an automobile, particularly to a seal between an outer peripheral surface of an outer housing of a constant velocity universal joint and an inner peripheral surface of a large-diameter-side end portion of a boot fixed to the outer peripheral surface.
2. Description of the Related Art
Constant velocity universal joints have been used, for example, in opposite end portions of a driving shaft for an automobile. Moreover, flexible boots which cover bent portions of the constant velocity universal joints are attached in order to seal grease for lubricating the constant velocity universal joints and prevent invasion of foreign matters such as dust and water from the outside. Large-diameter-side and small-diameter-side end portions of the boot are usually fastened and fixed to an outer peripheral surface of an outer housing (casing) of the constant velocity universal joint on a differential gear (differential) side or a hub side, and an outer peripheral surface of a drive shaft portion by bands, respectively.
In general, the casing whose outer peripheral surface has a simple cylindrical shape is used in the constant velocity universal joint on the hub side (outboard side).
In this case, the inner peripheral surface of the boot for the constant velocity universal joint, and the outer peripheral surface of the casing of the constant velocity universal joint are prevented from being shifted from each other, and are sealed, for example, by engagement of a protrusion protruded/disposed on the inner peripheral surface of the large-diameter-side end portion of the boot for the constant velocity universal joint with a peripheral groove disposed in the outer peripheral surface of the casing over a peripheral direction.
On the other hand, in the constant velocity universal joint on the differential side (inboard side), a tripod joint (triport joint) is generally used constituted by axially slidably disposing, for example, three sets of rollers on a shaft portion of the driving shaft in a trifurcated manner.
In order to achieve thinning and lightening, for example, three axial-direction grooves disposed in an axial direction of the outer peripheral surface are formed in the casing of the tripod joint in a scattered manner in the peripheral direction. In this case, for example, a thick portion whose shape viewed in the axial direction protrudes in a circular shape is formed in the inner peripheral surface of the large-diameter-side end portion of the boot for the constant velocity universal joint in such a manner as to be adapted to the groove surface of the axial-direction groove of the casing.
When the tripod joint is used in this manner, it is also considered that the outer peripheral surface of the casing and the inner peripheral surface of the boot are prevented from being shifted, and sealed by the engagement of the peripheral groove formed in the casing with the protrusion formed on the boot side in the same manner as in the outboard side.
However, when the groove of the peripheral direction is formed in the groove surface of the axial-direction groove in the casing outer peripheral surface, there is a problem that machining at the time of preparation of the casing becomes remarkably complicated and this increases costs.
To solve the problem, the following technical means for preventing the shifting and achieving the sealing on the differential side (inboard side) are known.
That is, a concave groove for fastening a band is disposed in the outer periphery of the large-diameter-side end portion, a plurality of circumferential seal lips are disposed on the large-diameter-side end portion inner periphery positioned right under the concave groove, the large-diameter-side end portion is fastened via fastening means such as a band from the outer peripheral side, and protruding end portions of the seal lips are pressed onto the outer peripheral surface of the casing to thereby achieve the sealing. Moreover, a shift preventive concave portion is disposed in a circumferential direction inside portions in which the seal lips are arranged in the axial direction of the boot, and a protruding portion to engage with the shift preventive portion is disposed on the outer peripheral surface end portion of the casing of the tripod joint to thereby prevent the shifting (e.g., FIG. 4, etc. of Japanese Utility Model Laid-Open No. 62-16541, FIG. 1, etc. of Japanese Patent Application Laid-Open No. 2003-202034).
However, these prior techniques have the following disadvantages.
{circle around (1)} Since the seal lips are disposed in positions deviating from the shift preventive portion in the axial direction, a boot volume is large, materials are wasted, and costs are raised. Moreover, an insertion property of the boot into the casing has been unsatisfactory.
{circle around (2)} Since the shift preventive portion is consciously disposed in a position deviating from a band fastening position in the axial direction, the shift toward the small-diameter-side end portion of the boot can be regulated, but a force for regulating the shift toward the large-diameter-side end portion is remarkably small.
Furthermore, the technique described in Japanese Patent Application Laid-Open No. 2003-202034 also has the following disadvantages.
{circle around (3)} In a structure in which the bead (protruding portion) is disposed in the axial-direction groove of the outer peripheral surface of the casing of the tripod joint, and the concave portion to engage with the bead is disposed in the inner periphery of the large-diameter-side end portion of the boot, the insertion property of the boot into the casing is further deteriorated by the presence of the bead.
{circle around (4)} After forging, lathe working is not performed. Therefore, concentricity of a core of the attached boot with the driving shaft present in the casing is deteriorated, and boot lifetime is shortened in many cases.
{circle around (5)} Since the concave portion to engage with the bead is also disposed in the thick portion of the large-diameter-side end portion, the concave portion constitutes an undercut at a forming time, mold releasing has been difficult, and forming properties have been unsatisfactory.
{circle around (6)} Since the bead remains to be formed by forging, dimensional precision fluctuation is larger than that in the lathe working. Unless the concave portion on the boot side is provided with backlash, the portion falls on the bead. Even when the bands are fastened, the seal does not work in some cases.
The present invention has been developed in consideration of the problem of the conventional technique, and an object thereof is to prevent shifting and enhance sealing properties between a boot for a constant velocity universal joint, having a thick portion formed to protrude from the inner peripheral surface of a large-diameter-side end portion, and the outer peripheral surface of a tripod joint casing, further enhance insertion properties of the boot into the casing, and reduce costs of boot materials, so that cost reduction, casing preparing cost reduction, and manufacturing facilitation are achieved.